1. Field of the Invention
This invention relates to a subterranean oil or gas well safety valve system utilizing a reference pressure dome charge chamber in a subsurface control used to actuate the safety valve, and to a seal assembly for maintaining the dome charge chamber at a desired reference pressure.
2. Description of the Prior Art
It is common practice to provide subterranean oil and gas wells, particularly offshore installations, with a downhole safety valve which automatically shuts in the event of a problem at the well head which might otherwise permit the release of large amounts of well fluids. Conventional well control safety valves are located in the upper portion of the well. Normally in offshore wells the safety valve will be mounted a short distance below the ocean bottom. Such downhole safety valves typically include ball type or flapper type valves which are opened or closed by applying or releasing pressure at the surface to control fluid in a separate control line extending to the surface of the well. A rupture of this separate control line will reduce the pressure acting to keep the safety valve open thus permitting the ball or flapper to close and shut off the well.
One problem with positioning a safety valve above exposed tubing is that the integrity of the well depends not only upon the safety valve but also upon the integrity of the tubing extending from the safety valve to the packer, which would normally be located in the vicinity of the producing zone. Thus if any leaks develop in the tubing between the packer and the safety valve these leaks will result in the escape of fluids from the tubing to the tubing-casing annulus. This invention presents a solution to this problem since the safety valve is mounted in or below the packer thus isolating the tubing above the packer from the formation fluid.
It is also a conventional practice in either safety valves or other downhole oil tools to use a dome charge or gas pressure chamber as part of the actuating mechanism of the tool. Often it is impossible to use a spring with a sufficient strength to balance hydraulic or other forces acting on some actuating member in the tool. It is conventional to use a gas at high pressure to provide forces greater than those which can be developed with a mechanical spring. These gas actuated tools are commonly referred to as dome charge tools, and the chamber containing the gas at an elevated pressure is referred to as a dome charge chamber or reference pressure chamber. The converse of this situation is a tool which uses a low-pressure or atmospheric pressure chamber into which a piston or other actuating member extends. Such a low pressure or atmospheric pressure chamber provides a known reference against which differential forces acting through the piston can be created. Unfortunately, both dome pressure charges and atmospheric pressure chambers have proved unreliable since it has been difficult to prevent leakage of the gas contained in the chamber or in formation fluids past seals in the tool. Thus the constant reference pressure desired cannot be maintained over time.
One possible solution to this problem is to provide a barrier fluid in the vicinity of elastomeric seals to prevent the passage of the gas past the seals. This solution was suggested in U.S. Pat. No. 3,561,473. This device employed a chamber containing a liquid such as silicon oil which would be relatively impervious to the passage of a gas in an atmospheric pressure chamber. Seals were located between the barrier fluid chamber and the atmospheric chamber. The seal would prevent leakage of the oil or barrier fluid into the atmospheric pressure chamber and the barrier fluid would prevent leakage of the gas through the barrier fluid chamber, thus maintaining the atmospheric pressure at a constant pressure. Although this tool would be effective to improve the performance of a tool containing an atmospheric pressure chamber, some leakage would still be possible. This arrangement would not be expected to perform as satisfactorily when a high pressure or dome charge is employed, since the high pressure gas after passing through intermediate elastomeric seals would tend to move through the barrier fluid to a region of lower pressure.